Hi Rhona - there are several keys to drilling without cracking the
glass:-

1. keep the glass cool by keeping it under water;

2. use a diamond drill bit (other types will barely scratch the
surface);

3. drill slowly - I use the traditional bow drill, although one can
use an electric one;

4. make sure the back of the glass is supported (for example on a
small block of wood) so when the hole is drilled through it doesn't
chip on the reverse. Even better, drill half way then reverse & meet
it the other way.